Structuring materials on a lower scale relative to the component size (mesoscale), e.g., µm-cm, enables the design of almost arbitrary effective physical properties in so-called metamaterials [1]. By controlling their non-linear mechanical behavior, the deformation of the material can be programmed [2]. Hereby, bistable elements can act as a switch between two states, that are characterized by a different shape or stiffness [3]. In traditional systems, bolted joints are typically used to alter the shape, or multiple product variants are utilized when different stiffness levels are needed.

The use of programmable materials allows for the creation of non-assembly components or the reduction of the number of variants by integrating functionality into the materials.

In this presentation, we will demonstrate the design process of such materials using two examples: one material that allows for 90° bending with multiple stable positions, and another that can switch between two effective stiffness levels. In the first design, different states were achieved by combining bistable elements with an additional global structure. The second design was enabled at the unit cell level.

Different base materials as well as limitations from the manufacturing process were considered. On one hand, a TPC (thermoplastic elastomer copolyester) filament was selected and processed with a standard fused filament fabrication printer, enabling large reversible shape changes due to its extensive elastic region and simple processing. On the other hand, a resin (durable resin, FLDUCL02) that yields stiffer material properties was processed using SLA printing to create more intricate three-dimensional structures (see Figure 1). We present numerical and experimental results for both designs, wherein the influence of geometrical parameters and base material is analysed. Especially, the combination of multiple bistable elements is challenging and can be tackled with a multiscale approach. Furthermore, the reliability of the structures is discussed based on cyclic compression tests.

References

[1] Kadic et al. 3D metamaterials, Nature Reviews Physics 1, 198-210, 2019.

[2] Wenz et al. Design of Shape Morphing Behavior through Local Programming of Mechanical Metamaterials, Advanced Materials, e2008617, 2021.

[3] Wenz et al. Controlling Malleability of Metamaterials through Programmable Memory, Advanced Engineering Materials, 2201022, 2022.